weather + forecasting

recipe for a weather forecast

1. initial conditions

2. data assimilation

3. numerical weather prediction

4. interpretation

chaos in modeling

• atm. is a chaotic fluid

• approximate present does not approximately determine future

Lorenz equations

system of 3 equations for a position of a particle in a space

• describes chaotic behavior of a particle

limit of prediction

• deterministic limit is 2 weeks

• set by chaos + uncertainty of the initial condition

model grid

• atm. divided into grid cells

• laws of physics evaluated for each cell in horizontal + vertical direction

explicit resolution

for scales larger than the grid scale

parameter

represent sub-grid effects/phenomenons

• contribute error to forecasts

data assimilation

process where observations are put into a model to generate initial conditions

• takes observations

• moves background field toward observations

background field

short-term forecast from previous run of model

ensemble

set of forecasts from slightly different starting conditions

• where most weather predictions come from

reliable forecast

observed frequency matches predicted frequency

probabilistic forecast

predicts climate not specific weather

cloud formation

around condensation nuclei from cooling + condensing

types of cloud formation

• air rising from solar heating

• air pushed up over mountains

• air lifted over a weather front

atmospheric stability

weather saturated air can freely rise

low clouds

• stratus

• cumulus

• stratocumulus

• cumulonimbus

medium clouds

• nimbostratus

• altostratus

• altocumulus

high clouds

• cirrus

• cirrostratus

• cirrocumulus

warm front

boundary between cold air ahead of front + warm air behind front

• associated with rain

clouds + climate

• high clouds trap more heat ↑ T

• low clouds reflect more sunlight ↓ T

climate

statistical distribution of weather over decades

what changes impact of extreme weather

• time

• climate

• conditions leading up to the event

• initial conditions

UK heatwave

T exceeds a regionally-varying threshold > 3 days

tropical cyclones

• classified based on sustained wind speeds

cyclone cone of uncertainty

shows avg. uncertainty, typical error of forecast based on the past 5 years

supercells

• intense, rotating thunderstorms under which tornadoes form

• requires unstable atm. + wind shear

in-situ atm. sensing

• weather stations

• buyos

• weather balloons

• aircraft

remote atm. sensing

• radar

• satellite

measuring T

• through Stevenson screens

• ground vs. air temp

hygrometer

humidity measuring device

relative humidity

actual water vapor/water vapor at sat.

dew point

T when air becomes fully sat. when cooled

wet bulb T

T of air becomes fully sat. when colled

absolute humidity

total water vapor in air

anemometer

measures wind @ 10 m

pyroheliometers

measures sunshine

radiosondes

• weather balloons

• only launched over land

satelite imagery for weather

• visible

• IR → cloud height

• water vapor → wavelengths absorbed by water

geostationary satellites

• orbit @ same speed as earth

• show evolution of weather systems

polar orbiting satellites

• orbit close to earth

• orbit 1 time daily